: Earth’s oxygen levels rose and fell more than once hundreds of millions of years before the planet-wide success of the Great Oxidation Event about 2.4 billion years ago, a study has found.

The work could help keep astronomers from unduly ruling out “false negatives,” or inhabited planets that may not at first appear to be so due to undetectable oxygen levels, the researchers said.

The researchers confirmed the appearance of oxygen in Earth’s past, roughly 150 million years earlier – or about 2.66 billion years ago – and lasting for less than 50 million years.

The team analysed drill samples taken by Roger Buick, a professor at the University of Washington, in 2012 at another site in the northwestern part of Western Australia called the Jeerinah Formation. Analysing successive layers in the rocks shows, Buick said, a “stepwise” change in nitrogen isotopes and then back again to zero.

The nitrogen isotopes reveal the activity of certain marine microorganisms that use oxygen to form nitrate, and other microorganisms that use this nitrate for energy.

The data collected from nitrogen isotopes sample the surface of the ocean, while selenium suggests oxygen in the air of ancient Earth. Koehler said the deep ocean was likely anoxic, or without oxygen, at the time. The team found plentiful selenium in the shallow hole only, meaning that it came from the nearby land, not making it to deeper water.

Selenium is held in sulphur minerals on land; higher atmospheric oxygen would cause more selenium to be leached from the land through oxidative weathering – “the rusting of rocks,” and transported to sea, Buick said.PTI

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